The present invention relates to a wide band and low noise amplifier for metric waves. It applies particularly to the construction of receivers operating close to transmitting antennae.
Conventionally this type of amplifier includes a transistor connected as common emitter, associated with a feed-back transformer between the output and the input of the amplifier, i.e. between the collector of the transistor and its base. Such a circuit makes it possible to obtain a high-frequency voltage at the terminals of the transformer, but the parasite capacitances of the feed-back windings limit the pass band.
A solution to this problem was described by the applicant in the French patent application No. 84 172 95. The amplifier which is there described comprises a transistor associated with a feed-back circuit, including two transformers. The primary of the first transformer is connected between the emitter of the transistor and its base, and the primary of the second transformer is connected between the collector of the transistor and the common potential point. The secondaries of both transformers are connected in series with the emitter circuit of the transistor. The output signal of the amplifier is taken from a part of the primary of the secondary transformer and the number of turns of both secondaries is very low so as to limit the leak inductance.
Thus a reverse gain of the amplifier is obtained practically identical to and of a sign opposite its forward gain. But because of the coupling of the source and load circuits, problems occur in matching the source and load circuits which are difficult to overcome especially when these circuits are formed by filters, the mismatching of the circuits causing modifications of the response curves.